An adaptive array antenna is an antenna having a plurality of antenna elements for adaptively controlling the directivity according to a radio-wave propagation environment to acquire a desired wave and suppress an interference wave. An adaptive array antenna is characteristic in that a main beam is directed to a desired direction by means of beam forming, and that a null point of a directivity pattern is formed in an interfering direction by means of null steering.
In general, it is known that an adaptive array antenna presents more favorable transmission/reception characteristic when correlation between transmission/reception signals in the respective antenna elements forming an array is smaller. As the correlation theoretically becomes smaller as the special distance between the antenna elements becomes larger, it is generally a case in which two or more antenna elements located sufficiently apart from each other are combined to thereby form a single adaptive array antenna.
However, in actuality, the correlation is changed due to various factors other than the spatial distance between the antenna elements. For example, a displaced antenna position due to inappropriate installation or the like, manufacturing variation or aged deterioration of an antenna element and a transmission/reception device including the antenna element, and so forth can be major causes to change the correlation between the antenna elements or transmission/reception devices. Further, leakage of an radio wave through a shielding gap in a transmission/reception device which is caused when a high frequency shielding in the transmission/reception device is manufactured with insufficient accuracy may also cause to change the correlation. Still further, change in the surrounding radio wave environment may also cause to change the correlation. When adaptive array transmission/reception is carried out using a combination of antenna elements or transmission/reception devices having correlation thereof deteriorated due to the above described causes, the adaptive array performance thereof, that is, beam directivity performance thereof, is remarkably deteriorated.
In view of the above, there is currently proposed a technique for determining an antenna element combination having the smallest correlation by calculating the correlation between antenna elements, based on actual measurement values of signals received at the respective antenna elements, rather than being premised on the theoretical correlation based on the spatial distance between the antenna elements.
For example, Patent Document 1, described below, discloses a technique, employed in an radio wave transmission/reception device equipped with a plurality of antenna elements having mutually different polarization directions and located spatially apart from one another, for determining an antenna element combination having the smallest correlation coefficient, based on signals received at the respective antenna elements, to thereby ensure sufficient transmission capacity, while reducing the antenna element correlation, even in a sequentially changing environment.
Also, Patent Document 2, described below, discloses a technique, employed in an adaptive array radio device having three or more antenna elements, for estimating respective antenna correlation values, based on the respective signals subjected to adaptive array reception by two selected antenna elements, and determining a combination of two antenna elements having the smallest antenna correlation value, to thereby improve the adaptive array receiving performance, while minimizing an addition of an antenna element and a transmission/reception device.    Patent Document 1: Japanese Patent Laid-open Publication No. 2004-312381    Patent Document 2: Japanese Patent Laid-open Publication No. 2004-289407